1. Field of the Invention
The present invention relates to a power management conversion output device, particularly to a single-inductor multi-output (SIMO) power management conversion output device.
2. Description of the Related Art
In the recent years, with the development and progress of IC fabrication process, the area of a chip is smaller and smaller and the chip has more and more functions. As a result, the volumes of all kinds of products are scaled down, such as mobile phones, handheld computers, digital media players and computers. The trend of the electronic products is toward to light weight and small volume, thereby producing portable electronic products.
In general, users expect that the products have small volumes, complete functions, strong performances and long operation time. In order to satisfy the requirement, circuits with different functions are installed in the product, and the circuits require different driving voltages. Since an external power supply only provides a fixed voltage, a DC converter in a power management device has to provide different output voltages for the circuits.
In practice, the internal circuits of the product perform different functions. As a result, the internal circuits require different voltages and load currents. A good power management device has to provide different output voltages, and each output voltage can provide enough large load current range to adapt to different operations. Accordingly, how batteries of portable electronic products possess the longest life and the greatest efficiency for use to achieve the greatest performance of the products and increase competitiveness is an important topic.
Further, a good power management device comprises a plurality of switched voltage converters. Power transistors and inductors of the converters occupy a very large area, which is a cost disadvantage. In the recent years, a single-inductor multi-output (SIMO) architecture is a popular solution to area occupation.
The SIMO architecture is different from the traditional DC to DC converter and decreases the amount of inductors, thereby saving the cost and improving competitiveness of products. It is apparent to reduce the area of print circuit boards without using inductors, thereby microminiaturizing portable products.
Nowadays, with the trend of integrating ICs, the power management device has to receive an input voltage to send out different voltages to different devices and circuits for use. By using the SIMO architecture, a power management device can convert power the most efficiently in cooperation with the least external inductors or capacitive elements. The power management device can provide stable output voltages and output currents required to efficiently use the batteries.
The SIMO architecture uses only one inductor and provides different voltages for multiple function circuits of a portable product. Although the SIMO DC to DC converter can save the chip area, the different output voltages still result in cross regulation. Cross regulation is more apparent when the conditions for the output load currents are different. Cross regulation becomes serious and affects the regulation effect when the output loads of the converter have greater difference.
However, the SIMO architecture still has the following problems:                (1) The more the amount of the outputs, the more the amount of interference sources causing cross regulation. Thus, it is harder to maintain the regulation.        (2) If the load range of the SIMO architecture is enlarged, the output voltages have to be stable in a light-load state or a heavy-load state. Limited by cross regulation, the converter can not operate in continuous conduction mode (CCM) in a heavy-load state. As a result, the heavy currents are limited not to enlarge the current range.        (3) When the SIMO architecture operates to reduce cross regulation, the output ripple voltages become higher.        
In addition, since a single-inductor one-output architecture simply has an output voltage, a control circuit adjusts one output voltage within one switched period. Since the SIMO architecture has two or more output voltages, a control circuit adjusts at least two output voltages within one switched period. Thus, the influence is described as the followings:                (1) When the amount of the output voltages increase, the time distributed to each output is not enough within one period to compress the time that any output voltage charges or discharge the inductor.        (2) When one output load changes, the charge and discharge time is also adjusted. Then, the final value of the inductor current can affect the next output voltage when the inductor discharges.        (3) The inductor energy provided by source power is too low to assign to each output.        
According to the abovementioned, when one output load of the SIMO converter changes, the control circuit has to adjust a duty cycle for the output load condition. However, during the adjustment process, the other output voltages not to need adjustment are possibly affected. The output voltages will be varied and unstable. The phenomenon that the voltage variation comes from the change of another output voltage is called cross regulation. Serious cross regulation leads to unstable output voltages.
Refer to FIGS. 1a-1d. FIG. 1a is a diagram schematically showing charge and discharge currents of an inductor of a SIMO architecture in the traditional technology. FIG. 1b is a diagram schematically showing charge and discharge currents of a light load and a heavy load of a single-inductor two-output architecture in the traditional technology. FIG. 1c is a diagram schematically showing charge and discharge currents of a single-inductor two-output architecture for a fixed time in the traditional technology. FIG. 1d is a diagram schematically showing charge and discharge currents of a single-inductor two-output architecture on continuous or discontinuous conduction threshold in the traditional technology. FIG. 1e is a diagram schematically showing charge and discharge currents of a single-inductor two-output architecture in energy-conservation mode in the traditional technology. An upper diagram and a lower diagram of FIG. 1a respectively show charge and discharge currents of the inductor for two outputs and four outputs. In FIGS. 1a-1e, IL denotes an inductor current, T denotes a switched period, and t denotes time.
Compared with two output voltages Vout1 and Vout2, the charge and discharge time of one of four voltages Vout1, Vout2, Vout3 and Vout4 are shortened (period T/2→period T/4) without changing period such that the time that the inductor current stores or discharges energy is shortened. In other words, when one of the four voltages Vout1, Vout2, Vout3 and Vout4 operates in a heavier-load state than a previous state, the charge time is shortened due to the fact that the period changes, thereby resulting in an inaccurate output value. Alternatively, the discharge time is too short to satisfy a stable condition of the inductor current
      [                            i          L                ⁡                  (                      T            4                    )                    =                        i          L                ⁡                  (          0          )                      ]    .Meanwhile, another output voltage has operated, which results in an output voltage error.
Refer to FIG. 1b which explains the abovementioned. Take the SIMO architecture for example. Suppose the time (such as T/2) that each output voltage Vout1 and Vout2 distributes the charge and discharge energy to the inductor is fixed, and the time (such as T/2) that the inductor current IL charges and discharges energy is fixed. When the output voltage Vout1 operates from the light-load state to the heavy-load state, the time that the inductor is charged needs to increase due to that the fact the output voltage Vout1 requires more energy. Thus, the time that the inductor current IL is shortened not to satisfy a stable condition of the inductor current
      [                            i          L                ⁡                  (                      T            2                    )                    =                        i          L                ⁡                  (          0          )                      ]    .Meanwhile, the output voltage Vout2 has been adjusted, which apparently interferes with an initial value of the inductor current IL of the output voltage Vout2.
The same phenomenon occurs in different charge and discharge mode of inductor energy. For example, the operation of FIG. 1c is different from that of FIG. 1a and FIG. 1b. In FIG. 1c, the time that the inductor current IL charges energy is fixed and different loads are discharged in order within other time of a period. As a result, in any case, the energy obtained by the charge of the inductor current IL is a fixed value in the first semi-period. In the second semi-period, the energy is distributed to each output voltage Vout1 and Vout2 in order. Therefore, when the load of the output voltage Vout1 changes, the energy obtained by the output voltage Vout2 is directly affected, thereby resulting in voltage variation. From FIG. 1c, the more the energy that the output voltage Vout1 requires, the longer the time that the inductor current IL discharges energy. However, the long time compresses the adjustment for Vout2.
Refer to FIG. 1d which proved that the influence on another output voltage when the output load changes beyond continuous or discontinuous conduction threshold. When the converter is in a stable state, the average currents are expressed by the equations 1-3:
                              I          OA                =                                            1              2                        ·                                          V                IN                            L                                ×                                                    M                A                            -              1                                      M              A              2                                ×                                    ϕ              A              2                                                      ϕ                A                            -                              ϕ                B                                                                        Equation        ⁢                                  ⁢        1                                          I          OB                =                                            1              2                        ·                                          V                IN                            L                                ×                                                    M                B                            -              1                                      M              B              2                                ×                                    ϕ              B              2                                                      ϕ                A                            +                              ϕ                B                                                                        Equation        ⁢                                  ⁢        2                                                      I            OA                                I            OB                          =                                            (                                                M                  B                                                  M                  A                                            )                        2                    ×                      (                                                            M                  A                                -                1                                                              M                  B                                -                1                                      )                    ×                                    (                                                ϕ                  A                                                  ϕ                  B                                            )                        2                                              Equation        ⁢                                  ⁢        3            
Wherein VIN is an input voltage, L is an inductor,
            M      A        =                  V        OA                    V        IN              ,            M      B        =                  V        OB                    V        IN              ,VOA and VOB are output voltages of two ends, φA is operation phase time of a load current Io1, and φB is operation phase time of a load current Io2. According to the equations of IOA and IOB, when the output current IOA (or IOB) increases whereby φA (or φB) varies over
      T    2    ,another output voltage will be affected.
According to the waveforms of the inductor currents, it is known that the continuous relations exist between the inductor current IL and the outputs, which results in cross regulation. In other words, the discontinuous relations exist between the inductor current IL and the outputs, which difficulty results in cross regulation.
Refer to FIG. 1e. In the traditional technology, a power stage energy-storage element stores energy, and then discharges the energy in order. The energy-storage element stores energy in two stage of energy-conservation mode (ECM). The energy obtained in the first storage stage are provided to the output voltage Vout1, and the energy obtained in the second storage stage are provided to the output voltage Vout2. Since the energy that each output voltage requires are independently distributed, the energy are enough to use. Even if the output voltage Vout1 requires large energy, the output voltage Vout1 does not seize the output voltage Vout2. As a result, the cross regulation can be greatly reduced. The energy obtained in the first storage stage is conserved until the energy-storage activity for another output voltage is finished, as shown in FIG. 1e. ΦA and ΦB are respectively intervals of the energy-storage activity for the output voltages Vout1 and Vout2. ΦC and ΦD are respectively intervals of the energy-discharge activity for the output voltages Vout1 and Vout2.
In ECM, the relation and the order of the energy-storage and energy-discharge activities of different output voltages make the allowable road range of each output not to be limited by cross regulation. In fact, the order of the power stage energy-storage and energy-discharge activities depends on the output loads. For the energy-storage activity of ECM, the adjustment activities are performed on the output voltages according the order from the lightest load closer to the heaviest load. However, when the adjustment order is fixed, the magnitude of the output loads changes, which still results in cross regulation.
To overcome the abovementioned problems, the present invention provides a single-inductor multi-output (SIMO) conversion device for enlarging load range, so as to solve the afore-mentioned problems of the prior art.